Method and apparatus for resistivity imaging in boreholes filled with low conductivity fluids

ABSTRACT

A mixed mode tool uses an inductive source and detects galvanic currents and/or potentials at electrodes in proximity to a borehole wall to produce a resistivity image of the earth formation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to exploration for hydrocarbons involving electrical investigations of a borehole penetrating an earth formation. More specifically, this invention relates to highly localized borehole investigations employing the introduction and measuring of individual survey currents injected into the wall of a borehole by capacitive coupling of electrodes on a tool moved along the borehole with the earth formation.

2. Background of the Art

Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a measure electrode (current source or sink) are used in conjunction with a diffuse return electrode (such as the tool body). A measure current flows in a circuit that connects a current source to the measure electrode, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present invention is a hybrid of the two.

There are several modes of operation of prior art devices: in one, the current at the measuring electrode is maintained constant and a voltage is measured while in the second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant the voltage measured at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if this current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. Ohm's law teaches that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.

Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Patent No. 685727 to Mann et al., U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. and U.S. Pat. No. 6,714,014 to Evans et al, each of which provide additional background information to this disclosure.

In the prior art devices, current is actively focused in the direction perpendicular to the borehole wall. There is a technical challenge to provide stable focusing conditions during the logging if the borehole walls are rough or the mud is very conductive. As soon as the focusing conditions are not met, the measurements are responsive to a considerable extent to the properties of the mud. The prior art devices do not specifically address the problems due to irregularities in the wall surface of the wellbore. If the wall of the wellbore is irregular, the measuring current path becomes distorted and the relationship between measured impedance and earth formation resistivity changed as result.

SUMMARY OF THE INVENTION

One embodiment of the invention is an apparatus for evaluating an earth formation. The apparatus includes at least one coil on a logging tool conveyed in a borehole in the earth formation. Passage of a current through the coil induces an electrical current in the earth formation. At least two electrodes associated with the logging tool and in proximity to a wall of the borehole have a potential difference that is indicative of a property of the earth formation. The at least one coil may be mounted on a mandrel of a downhole assembly. The at least one coil may include at least three planar coils with their normals distributed azimuthally about an axis of the logging tool. At least one of the three coils may operate at a different frequency from another one of the at least three coils. The electrodes may be positioned on a first pad extendable from a mandrel of the downhole assembly. A second pad may be provided on an opposite side of the mandrel from the first pad. The two electrodes may further include three pairs of electrodes, each pair being at a different azimuthal position on the pad. The apparatus may further include a processor which uses the difference to estimate a resistivity property of the earth formation. The logging tool may be conveyed into the borehole using a wireline, a drilling tubular and/or a slickline. The difference may be in quadrature with the current in the coil.

Another embodiment of the invention is a method of evaluating an earth formation. A current is passed through at least one coil on a logging tool conveyed in a borehole in the earth formation so as to induce an electrical current in the formation. A difference in electrical potential between at least two electrodes in proximity to a wall of the borehole is detected.

Another embodiment of the invention is a computer-readable medium for use with an apparatus for evaluating an earth formation. The apparatus includes at least one coil on a logging tool conveyed in a borehole in the earth formation. Passage of current through the coil induces an electrical current in the earth formation. The apparatus also includes two electrodes in proximity to a wall of the borehole. The medium includes instructions which enable a processor to determine from a difference in electrical potential between the two electrodes a property of the earth formation. The medium may include a RAM, a ROM, an EPROM, an EAROM, a flash memory, and/or an optical disk.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is best understood with reference to the accompanying figures in which like numerals refer to like elements and in which:

FIG. 1 shows an exemplary logging tool suspended in a borehole;

FIG. 2 is a mechanical schematic view of an exemplary imaging tool;

FIG. 3 a is a schematic illustration of three coils on a tool of the present invention;

FIG. 3 b illustrates an embodiment of the present invention showing a single coil on a mandrel and pad mounted electrodes;

FIG. 3 c is an equivalent circuit diagram of a resistivity imaging tool;

FIGS. 4 a and 4 b shows an arrangement of pad mounted electrodes and a pad mounted coil;

FIGS. 5 and 6 show exemplary models used for evaluation of the tool configuration of FIG. 3 b;

FIG. 7 shows the response at different azimuths for the model of FIG. 5 with no rugosity;

FIG. 8 shows the K-factor for the curves of FIG. 7 for azimuths of 10° and 20°;

FIG. 9 shows the results of applying the K-factor to the 10° and 20° azimuth curves;

FIG. 10 shows the response at different standoffs for the model of FIG. 5; and

FIG. 11 shows the effect of rugosity on measurements made by an exemplary tool. of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an imaging tool 10 suspended in a borehole 12, that penetrates earth formations such as 13, from a suitable cable 14 that passes over a sheave 16 mounted on drilling rig 18. The cable 14 includes a stress member and seven conductors for transmitting commands to the tool and for receiving data back from the tool as well as power for the tool. The tool 10 is raised and lowered by draw works 20. Electronic module 22, on the surface 23, transmits the required operating commands downhole and in return, receives data back which may be recorded on an archival storage medium of any desired type for concurrent or later processing. The data may be transmitted in analog or digital form. Data processors such as a suitable computer 24, may be provided for performing data analysis in the field in real time or the recorded data may be sent to a processing center or both for post processing of the data.

FIG. 2 a is a schematic external view of a borehole sidewall imager system. The tool 10 comprising the imager system includes resistivity arrays 26. Optionally, the imager system may include other sensors, such as a mud cell 30 or a circumferential acoustic televiewer 32. Electronics modules 28 and 38 may be located at suitable locations in the system and not necessarily in the locations indicated. The components may be mounted on a mandrel 34 in a conventional well-known manner. The outer diameter of the assembly may be about 5 inches and about fifteen feet long. An orientation module 36 including a magnetometer and an accelerometer or inertial guidance system may be mounted above the imaging assemblies 26 and 32. The upper portion 38 of the tool 10 contains a telemetry module for sampling, digitizing and transmission of the data samples from the various components uphole to surface electronics 22 in a conventional manner. If acoustic data are acquired, they are preferably digitized, although in an alternate arrangement, the data may be retained in analog form for transmission to the surface where it is later digitized by surface electronics 22. Also shown in FIG. 2A are three resistivity arrays 26 (a fourth array is hidden in this view.

Turning to FIG. 3 a, a simplified exemplary diagram of three vertical coils 101, 103, 105 of the present invention on a mandrel (not shown) of the present invention is shown. FIG. 3 b show one of the coils 103 mounted inside or on the surface of mandrel 121. Arms depicted schematically by 131, 137 extend a pad 133 radially outward from the mandrel to make contact with the borehole wall (not shown). Disposed on the pad 133 are electrodes depicted schematically by 135 a, 135 b. Another pad (not shown) may be positioned on the opposite side of the coil 103 from the pad 133. In an alternate embodiment of the invention, a single coil may be mounted on the mandrel with its axis along the tool axis.

This tool may be referred to as a “mixed mode” tool in that an inductive source is used and galvanic currents are detected by the electrodes. Specifically, a plurality of long transversal rectangular coils with the magnetic moment perpendicular to the axis of the borehole are used. Each transmitter loop is centered in the borehole and electrode pairs are placed on the pad attached to the borehole wall. This is a generic design and further variants are identified below. In a practical design each transmitted coil serves two pads with a number of electrode pairs on each pad. Each transmitter coil may have its own operating frequency to avoid the interference of the neighboring induction coils. By using an induction transmitter, an electric current can be injected into the formation.

(1),

At a low frequency and relatively close to the induction loop, the electric field does not depend on the conductivity of the formation and can be increased simply by increasing the operating frequency ω. In the case of a galvanic injection and non-conductive mud the injection current must go through quite a large capacitive resistance. This can be better understood from the simplified schematics in FIG. 3 c where the capacitor C_(m) represents the capacitance between the injection electrodes and the formation, and R_(f) corresponds to the resistivity of the formation. The current I_(f) injected into the formation can then be expressed as

$\begin{matrix} {{I_{f} = \frac{U_{ab}}{{2X_{c}} + R_{f}}},\mspace{31mu} {X_{c} = \frac{1}{i\; \omega \; C_{m}}},\mspace{31mu} {C_{m} \approx {ɛ\frac{S}{d}}},} & (2) \end{matrix}$

where S is the area of the electrode, U_(ab) is the applied potential difference between the injection electrodes a and b. Because C_(m) is inversely proportional to the distance d between the current electrode and the formation, the amount of the current injected into the formation will drop with increasing standoff. A long induction transmitter is free of such high sensitivity to the standoff value and well suited to the nonconductive environment.

If only electric field is measured, the measurements will be very sensitive to a relative variation of resistivity in the adjacent formation. To derive the absolute resistivity of the formation, some additional induction measurements and their combination with the galvanic readings are helpful.

The response of the tool design of FIG. 3 b was tested on a number of different models. One of these is illustrated in FIG. 5. Shown therein is a borehole 151 with a diameter of 8.5 inches (21.6 cm). The mandrel is shown as 121, a pad by 132 and an arm on which the pad is carried by 131. The tool has a variable standoff 133. The formation comprises beds of thickness 0.5 inches, 1 inch, 2 inches, 3 inches and 4 inches (1.27 cm, 2.54 cm, 5.08 cm, 7.62 cm and 10.16 cm respectively). The layers had resistivities ρ and relative dielectric constant ∈ of (10 Ω-m, 10) and (1 Ω-m, 20) respectively.

In a second model shown in FIG. 6, the formation had a uniform ρ=1Ω-m, ε=20, The standoff was fixed at ⅛ inches (3.18 mm). However, the borehole was rugose, with a depth of rugosity varied between ¼ inches and ¾ inches (6.35 mm and 19.1 mm). Response to other models which represented a combination of the features of the models of FIGS. 5 and 6 were also simulated.

In the modeling, a 0.914-m long transmitter with a width of 0.1524 m was used. The operating frequency was 100 kHz. In the case of lower or higher frequencies (up to several MHz), the response can be approximately derived simply by linear resealing of the signal corresponding to 100 kHz frequency. A transmitter loop is placed in the nonconductive borehole environment with the radius of the borehole 10.795 cm. An electrode spacing of 0.25 inches or 0.5 inches (0.63 cm and 1.27 cm) was used to measure a potential drop U_(z) in the vertical direction parallel to the borehole axis.

The typical behavior of the electrical signal to the model is presented in FIG. 7. The three curves 201, 203 and 205 in this figure correspond to azimuths of 0°, 10° and 20° of the receiver's electrode pairs. The abscissa in the figure is the logging depth in inches and the ordinate is the signal (the voltage difference between the button electrodes). The 10° and 20° deg. azimuth curves can be shifted to the 0° deg. curve by applying a K-factor that is about 1.07 for the 10° curve and 1.27 for the 20-degree curve. The division result of 10° and 20° curves by the 0°. curve is presented in FIG. 8 as the curves 221 and 223, while the result of K-factor application to the original curves from FIG. 7 is presented in FIG. 9. 241 is the original 0° azimuth response to the model from FIG. 7. 243 is the corrected 10° azimuth response to the model while 245 is the corrected 20° azimuth response. From FIG. 9 we it can be seen that it is possible to cover an azimuth range of 40° (from −20° to +20°) by having additional columns of electrodes on the pad of FIG. 3 b.

The dynamic range, which is the ratio between the maximum and minimum reading along the logging depth, is changing between 5 and 6 considering layers 1 in. and larger. We define a Normalized Dynamic Range (NDR) as a ratio of a signal dynamic range to a resistivity contrast of the corresponding media. In the model of FIG. 5 the resistivity contrast of the neighboring layers is 10, so that the NDR of the mixed mode arrangement is approximately 0.55.

Next, examples showing the influence of the distance between the receiver electrodes and the borehole wall are presented. The results of mathematical modeling for the same benchmark model of FIG. 5 are presented in FIG. 10. The electrode spacing is 0.25 inches (6.35 mm). For the ⅛ inch (3.18 mm) standoff 263 the NDR drops to 0.3 in the 1 inch (2.54 cm) thick layer and decreases to 0.2 and 0.13 for the ¼ in (6.35 mm) 265 and ½ in (1.27 cm) 267 standoff, correspondingly. For the 2 in (5.08 cm) layer thickness the NDR parameter is as much as twice larger than for 1 in (2.54 cm) layers. The imperfections due to standoff are more noticeable in the conductive layers, and there is no signal imperfection in the resistive layers thicker than 2 in (5.08 cm).

Turning next to FIG. 11, the sensitivity of the measured electric field for the model of FIG. 6 as a function of borehole rugosity is shown. The curves 281, 283 and 285 correspond to rugosity of ¼ inch (6.3 mm), ½ inc (1.27 cm) and ¾ inches (1.91 cm) respectively. This is a model with no resistivity contrasts, but the signal from the rugose wall has all the features of a structure—boundaries and resistivity contrast. Of course, these artifacts are more pronounced for the ½ inc (1.27 cm) and ¾ inches (1.91 cm) rugosity than for the ¼ inch (6.3 mm) rugosity. Based on extensive modeling results (not shown), we have concluded that in the case of a 0.25-in. rugosity depth, all 1-in. beds are well resolved (NDR>0.2) and the presence of the rugosity in some beds does not destroy the readings in front of neighboring beds. The situation deteriorates as the rugosity is increased to 0.5 in. and 0.75 in.

The processing of the data may be done with the use of a computer program implemented on a suitable machine readable medium that enables the processor to perform the control and processing. The term processor as used in this application is used in its traditionally-broad sense and is intended to include such devices as single-core computers, multiple-core computers, distributed computing systems, field programmable gate arrays (FPGAs) and the like. The machine readable medium referenced in this disclosure is any medium that may be read by a machine and may include magnetic media, RAM, ROM, EPROM, EAROM, flash memory and optical disks. The processing may be done downhole or at the surface. In an alternative embodiment, part of the processing may be done downhole with the remainder conducted at the surface.

The invention has been described with reference to a wireline conveyed logging tool. The principles discussed above may also be used in a measurement-while-drilling (MWD) implementation in which the logging tool is part of a bottomhole assembly (BHA) conveyed on a drilling tubular. The method may also be used with the logging tool conveyed on a slickline. For the purposes of the present invention, the term “downhole assembly” may be used to describe a BHA as well as configurations in which the logging tool is part of an assembly conveyed on a wireline or slickline.

The following definitions are helpful in understanding the present invention.

coil: one or more turns, possibly circular or cylindrical, of a current-carrying conductor capable of producing a magnetic field;

-   EAROM: electrically alterable ROM; -   EPROM: erasable programmable ROM; -   flash memory: a nonvolatile memory that is rewritable; -   induction: based on a relationship between a changing magnetic field     and the electric field created by the change; -   machine readable medium: something on which information may be     stored in a form that can be understood by a computer or a     processor; -   mandrel: A bar, shaft or spindle around which other components are     arranged or assembled. The term has been extended in oil and gas     well terminology to include specialized tubular components that are     key parts of an assembly or system; -   misalignment: the condition of being out of line or improperly     adjusted; -   Optical disk: a disc shaped medium in which optical methods are used     for storing and retrieving information; -   Position: an act of placing or arranging; the point or area occupied     by a physical object -   ROM: Read-only memory; -   Resistivity: electrical resistance of a conductor of unit     cross-sectional area and unit length. Determination of resistivity     is equivalent to determination of its inverse (conductivity); -   Rugosity: A qualitative description of the roughness of a borehole     wall. Alternatively, the term pertains to a borehole whose diameter     changes rapidly with depth.

While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure. 

1. An apparatus for evaluating an earth formation, the apparatus comprising: (a) at least one antenna configured to be conveyed in a borehole in the earth formation and induce an electrical current in the earth formation upon activation of the antenna; and (b) at least two electrodes in proximity to the formation, a difference in electrical potential between the at least two electrodes being indicative of a property of the earth formation.
 2. The apparatus of claim 1 wherein the at least one antenna is mounted on a mandrel of a downhole assembly.
 3. The apparatus of claim 2 wherein the at least one antenna further comprises at least three planar coils with normals distributed azimuthally about an axis of a logging tool.
 4. The apparatus of claim 3 wherein at least one of the at least three coils operates at a different frequency from another one of the at least three coils.
 5. The apparatus of claim 1 wherein the at least two electrodes are positioned on a first pad extendable from a mandrel of a downhole assembly to the proximity of the formation.
 6. The apparatus of claim 5 further comprising a second pad disposed on an opposite side of the mandrel from the first pad, the second pad having at least two electrodes.
 7. The apparatus of claim 5 wherein the at least one antenna is disposed on the first pad.
 8. The apparatus of claim 5 wherein the at least two electrodes further comprise at least three pairs of electrodes, each of the pairs of electrodes being in a different azimuthal position on the first pad.
 9. (canceled)
 10. The apparatus of claim 1 further comprising a processor configured to determine from the difference in electrical potential a resistivity property of the earth formation.
 11. The apparatus of claim 1 wherein the at least one antenna is configured to be conveyed into the borehole using one of (i) a wireline, (ii) a drilling tubular, and (iii) a slickline.
 12. The apparatus of claim 1 wherein the difference is in quadrature with a current in the at least one antenna.
 13. A method of evaluating an earth formation, the method comprising: (a) passing a current through at least one antenna conveyed in a borehole in the earth formation, and inducing an electrical current in the earth formation; and (b) measuring a difference in electrical potential between at least two electrodes in proximity the formation resulting from the induced current, and (c) determining from the difference a resistivity property of the earth formation.
 14. The method of claim 13 further comprising positioning the at least one antenna on a mandrel of a downhole assembly.
 15. The method of claim 14 wherein the at least one antenna comprises at least three planar coils with normals distributed azimuthally about an axis of a logging tool.
 16. The method of claim 15 further comprising operating at least one of the at least three coils at a different frequency from another one of the at least three coils.
 17. The method of claim 13 further comprising positioning the at least two electrodes on a first pad extendable from a mandrel of a downhole assembly.
 18. The method of claim 17 further comprising disposing a second pad having at least two electrodes on an opposite side of the mandrel from the first pad.
 19. The method of claim 17 further comprising positioning the at least one antenna on the first pad.
 20. The method of claim 17 wherein further comprising: (i) using, for the at least two electrodes at least three pairs of electrodes, each of the pairs of electrodes being in a different azimuthal position on the first pad; and (ii) the method further comprising determining an image of the property of the formation.
 21. The method of claim 13 further comprising conveying the at least one antenna into the borehole using one of (i) a wireline, (ii) a drilling tubular, and (iii) a slickline.
 22. The method of claim 13 wherein the measured difference is in quadrature with the current.
 23. A computer readable medium for use with an apparatus for evaluating an earth formation, the apparatus comprising: (a) at least one antenna configured to be conveyed in a borehole in the earth formation, and induce an electrical current in the earth formation upon activation of the antenna; and (b) at least two electrodes and in proximity to a wall of the borehole; the medium comprising instructions which enable a processor to determine from a difference in electrical potential between the at least two electrodes a property of the earth formation.
 24. The medium of claim 23 further comprising at least one of (i) a RAM, (ii) a ROM, (iii) an EPROM, (iv) an EAROM, (v) a flash memory, and (vi) an optical disk.
 25. The apparatus of claim 10 wherein the processor is further configured to record the resistivity property on a suitable medium. 